1. Field of the Invention
This invention relates generally to the field of subsea drilling and in particular to a wellhead arrangement for use in drilling through shallow formations beneath the sea bed which are water bearing and under pressure.
2. Description of the Prior Art
A common subsea drilling technique involves first providing a large diameter hole and installing a conductor casing typically 36xe2x80x3 in diameter through the mud base of the seabed. Typically a low pressure or external wellhead housing is secured to the top of the conductor casing. Next, the well is bored through shallow earth formations to accept 26xe2x80x3 casing. The 26xe2x80x3 casing is installed in the hole with an internal or high pressure wellhead housing connected at its top and extends upwardly from the 26xe2x80x3 casing. The 26xe2x80x3 casing is cemented into the borehole through the use of a drill pipe conveyed cementing tool to the bottom of the hole. Cement is applied in the annulus between the 26xe2x80x3 casing and the borehole and up through the annulus between the 26xe2x80x3 casing and the 36xe2x80x3 conductor pipe. The cement returns are through flow ports in the external wellhead. The flow ports remain open after the 26xe2x80x3 casing is installed. Typically the 26xe2x80x3 hole extends down to about 1,500 or 2,000 feet.
The drilling then proceeds through the 26xe2x80x3 casing. A high pressure internal wellhead housing, a blowout preventer and a drilling riser are then installed. Two or more strings of casings are usually installed to line the borehole as it is drilled deeper through earth formations. Such strings of casings are landed and sealed in the internal wellhead housing. Such stings are cemented in place as described above with cementing tools landed in the wellhead housing and extending to the bottom of the casing.
The drilling procedure described above encounters problems where subsea formations include a shallow water flow zone, typically between 500 and 2,000 feet below sea bed. Such formations are water bearing and under pressure which exceeds sea floor water pressure by about 50 to 300 psi. When a 26xe2x80x3 borehole is drilled through such shallow water flow zone, the pressurized formation water will find any upward path through the cement of the annulus, about the 26xe2x80x3 casing and flow out the cement flow path of the external wellhead. Dangerous conditions may result from such flow at the sea bed. The well could become washed out.
Various solutions have been proposed to solve the problem of drilling through shallow water flow zones, typically found in the Gulf of Mexico. One solution is to use a foaming cement which retards washout.
U.S. Pat. No. 5,184,686 describes a system for avoiding washout, but it uses risers of two different diameters at various stages of drilling. The procedure is time consuming and expensive.
One prior system provides a ball valve in the flow ports of the external wellhead housing which may be closed by a Remotely Operated Vehicle (ROV) after the internal wellhead housing is provided. Closing the ball valves prevents shallow water flow zone water from leaking past the cemented annulus between the 26xe2x80x3 casing and the external wellhead housing secured to the top of the 36xe2x80x3 conductor pipe. Ball valves are expensive, add to operating difficulties and must be operated by means of an ROV.
U.S. Pat. No. 5,660,234 describes another prior system for solving problems associated with drilling through shallow water flow zones. The well is formed to a first depth, and 36xe2x80x3 conductor pipe is installed with an external wellhead housing located at its upper end and extending above the sea bed. A reciprocating valve sleeve is mounted above the flow ports on the external casing. The well is drilled to a second depth at a level which is above the water flow zone. A string of casing is installed in the base, supported by a scab hanger and cemented into this section of the well. Typically such casing is 26xe2x80x3 in diameter. This section of the well extends to a distance of about 300 feet about the water flow sand zone. The well is next drilled with a small diameter through the water flow sand. After drilling, the hole is swabbed with a foaming type cement to build up mudcake and retard washout. The well is then reamed to accept a smaller diameter casing, typically 20xe2x80x3 in diameter. The 20xe2x80x3 diameter casing is then run with a high pressure or internal wellhead housing located at its upper end. A running tool is used which latches to the external casing. The 20xe2x80x3 casing is cemented into the hole with cement returns flowing out the open flow ports of the external wellhead housing. Once cementing is completed, the running tool is used to move the valve sleeve to the closed position, thereby closing the flow ports. The operator retrieves the running tool and installs a blowout preventer and drilling riser to the internal wellhead housing The well is then bored to greater depths with at least two casing strings installed. A monitoring valve is mounted in a monitoring port in a section of the conductor pipe between the landing sub and the external wellhead casing. A remote operated vehicle must be used to monitor the valve to determine whether or not pressure has built up in the annulus about the 26xe2x80x3 casing.
A major disadvantage of the system described above is that it does not provide an indication, at the time of closing the valve sleeve, as to whether or not the shallow water flow ports are closed.
Another disadvantage of such system is that if the valve sleeves are faulty, they are not retrievable and replaceable, because they are part of the external wellhead housing
Another disadvantage of such system is that the valve sleeve is not run independently of the external wellhead housing.
Still another disadvantage of the above system is that the efficacy of the closing of the flow ports must be sensed by a ROV, rather than remotely from a service work vessel.
Identification Of Objects Of The Invention
A primary object of this invention is to provide a wellhead system for shallow water flow zone drilling in which a replaceable pack-off device is used to seal off ports for shallow water zone return flows.
Another primary object of this invention is to provide a wellhead arrangement by which a feedback signal is produced at a surface vessel via a hydraulic flow path from the wellhead to indicate whether or not the pack-off device is properly set.
Another important object of the invention is to provide a wellhead arrangement for shallow water flow zone drilling in which a pack-off device is run at the same time as is an internal high-pressure wellhead housing with the 20xe2x80x3 casing extending through the shallow water flow zone.
Another important object of the invention is to provide a running tool and method by which (1) a pack-off is set to close shallow water return flow ports from an annulus between external and internal wellhead housings, (2) the position of such pack-off is sensed remotely at the drilling vessel, (3) the pack-off is energized hydraulically from the drilling vessel, and (4) the pack-off can be replaced if a problem develops with the operation of the pack-off.
The objects identified above, as well as other advantages and features of the invention are embodied in a system which provides a cement return path in an annulus formed between the external housing (called the 26xe2x80x3 housing because it is secured to the 26xe2x80x3 casing) and an internal housing of 18xc2xexe2x80x3 internal diameter (secured to 20xe2x80x3 casing, but called herein as the 18xc2xexe2x80x3 housing). The 26xe2x80x3 housing connects to 26xe2x80x3 pipe cemented in a borehole above a shallow water flow zone. The 18xc2xexe2x80x3 housing is run, simultaneously by means of a running tool, with 20xe2x80x3 casing, a seal assembly, and cementing equipment through a bore drilled through the zone of shallow water pressurized flow. The retrievable seal assembly is placed in the annulus between the 26xe2x80x3 and 18xc2xexe2x80x3 housings initially above the cement return flow ports. The running tool and subsea wellhead assembly includes devices for selectively forcing, via a hydraulic path from a service vessel a seal or pack-off of the seal assembly below the cement return flow ports, thereby sealing the annular space from formation water flow between the 26xe2x80x3 and 20xe2x80x3 casings. After the pack-off is set, the running tool is returned to the surface, and drilling and casing operations of the well continues through the wellhead system. If a problem were to develop with the seal, a running tool is provided for retrieving and replacing the pack-off
The running tool and seal assembly are designed in order to force the pack-off of the seal assembly downward into the annulus below the flow ports with hydraulic pressure from the drill string forcing a piston downward against the seal assembly. A feedback mechanism is provided which generates a pressure signal at the surface vessel, via the hydraulic path, which is representative of the position of the pack-off in the annulus.